1. Field of the Invention
This invention relates to a system for non-coherently receiving and detecting data which has been modulated onto a carrier signal. More particularly, the present invention discloses a novel apparatus for detecting data signals modulated onto a carrier without the requirement for coherent demodulation of the carrier signal.
2. Description of the Prior Art
Digital data may be phase modulated onto a carrier signal before being transmitted. The process of modulating the data signal onto the carrier signal causes phase reversal of the carrier signal making it impossible to determine the absolute phase of the carrier.
Purely coherent receiving systems provide a replica signal of the carrier signal which will permit the determination of the absolute phase of the carrier signal and thus the absolute phase of the data signal with maximum effectiveness.
In one type of prior art system known as a differentially coherent system, the data which is employed to modulate the carrier signal at the transmitter is encoded at the transmitter and then decoded at the receiver without the requirement for the determination of the phase of the carrier. It is known that differentially coherent receiving systems are less efficient than a purely coherent receiving system by approximately two decibels.
In yet another form of non-coherent receiving system, the data modulated carrier signal is encoded at the transmitter. The received encoded signals are applied to a pair of double-bit matched filters at the receiver which operate to detect and decode the incoming signal. On the basis of the absolute value of the output from the pair of double-bit matched filters, a hard or final decision is made whether the received data bit signal is representative of the presence or absence of a transition in the data stream. Systems employing double bit matched filters at the receiver also typically cause a two decibel loss of the received signal when compared to a coherent receiver.
It would be desirable to provide a non-coherent receiving system which is more efficient than the differentially coherent system receiver and other forms of non-coherent receivers known in the prior art.